Does drinking water help you lose weight?

2026-10-30

Drinking water can help with weight loss, but modestly and indirectly. In two randomized trials, adults who drank about 500 ml (17 fl oz) of water before meals lost roughly 1–2 kg (2–4 lb) more over 12 weeks than those who didn't. The biggest and most reliable effect comes from something simpler: replacing sugary drinks with water, which removes calories without making you hungrier. Water doesn't burn fat, "flush" anything out, or meaningfully speed up your metabolism.

What the studies actually found

Water before meals

The best-known trial comes from Virginia Tech. Dennis and colleagues (2010) put 48 adults aged 55–75 with overweight or obesity on a calorie-reduced diet. Half of them also drank 500 ml of water before each of their three daily meals. After 12 weeks, the water group had lost about 2 kg more than the diet-only group. The authors suggested that the water reduced how much people ate at the following meal.

A larger trial in English general practices, Parretti and colleagues (2015), tested the same idea in 84 adults with obesity: 500 ml of water 30 minutes before main meals versus a control group asked to imagine a full stomach. After 12 weeks, the water group had lost 1.3 kg more. Once the results were adjusted for age, sex, ethnicity and deprivation, the difference was 1.2 kg and no longer statistically significant. The authors called the evidence "preliminary".

So the honest summary is: pre-meal water seems to help a little, probably by taking the edge off appetite, and the trials are small and short. It's a cheap, low-risk habit, not a weight-loss method on its own.

The "water boosts your metabolism" claim

You may have read that drinking 500 ml of water raises your metabolic rate by 30%. That figure comes from a small 2003 study. When Brown and colleagues (2006) repeated the experiment with careful measurements, room-temperature water did not increase energy expenditure at all, and ice-cold water raised it by only about 4.5% for an hour — a handful of calories. Drinking cold water to "burn calories" isn't worth the effort.

Where water really makes a difference: what it replaces

Water has zero calories. Many of the drinks it can replace don't. According to the CDC's "Rethink Your Drink" guide, a 12 fl oz (355 ml) can of regular soda has around 150 calories and about 10 teaspoons of added sugar. Sweetened iced teas, energy drinks, flavored coffees and many smoothies are in the same range or higher.

Liquid calories are easy to underestimate because they don't fill you up the way food does. If you drink one can of soda a day, swapping it for water removes roughly 1,000 calories a week. That won't transform your weight on its own, but over months it's one of the most effective single changes you can make, and it doesn't require eating less.

If you're not sure which of your drinks count toward hydration and which add significant calories, see do coffee, tea, milk and juice count toward your water goal?

How to use water in a weight-loss plan

Swap first. List the sweetened drinks you have in a typical day and replace one at a time with water, sparkling water, or unsweetened tea or coffee. Start with the one you'll miss least.
Drink a glass before meals. About 500 ml (two glasses, 17 fl oz) 30 minutes before main meals is what the trials used. If that's too much, a large glass still helps you start the meal less hungry.
Keep water within reach. A bottle on your desk or in your bag makes water the easy default when you're thirsty, rather than whatever is in the vending machine.
Make it enjoyable. Lemon, cucumber, mint, or sparkling water can replace the habit of a flavored drink. There are more ideas in don't like plain water?
Pair it with the basics. Water supports a calorie deficit; it can't replace one. The weight-loss results in both trials happened alongside a reduced-calorie diet or dietary advice.

Realistic expectations

Expect a small extra effect. Based on the trials, pre-meal water might add 1–2 kg of loss over three months. Swapping sugary drinks can add more, depending on how much you drank before.
Don't confuse water weight with fat. Your weight on the scale can move 1–2 kg from day to day with fluid, salt and carbohydrate intake. Drinking more water does not make you "retain" more water in a healthy person; extra fluid is simply passed as urine.
More isn't better. There's no benefit in forcing down large volumes. Drinking a lot of water very quickly can be dangerous, especially during endurance exercise; read can you drink too much water? for the limits.
Thirst and hunger are different signals. You'll often read that people "mistake thirst for hunger". The evidence for this is thin. A glass of water before a snack is a reasonable experiment, but don't rely on it to control appetite.

Who should check with a doctor

If you have heart failure, kidney disease or another condition where your fluid intake is limited, don't increase how much you drink without medical advice. The same goes for anyone taking medication that affects fluid balance, such as diuretics.

Making it a habit

Most people fail at "drink more water" not because they don't want to, but because they forget. A goal and a gentle nudge help. Water Tracker suggests a daily goal based on your weight and activity, lets you log a glass in one tap from your iPhone, Apple Watch or a Home Screen widget, and sends reminders that stop once you've reached your goal. If you're trying the pre-meal habit, logging the glass before lunch and dinner is an easy way to see whether you're actually doing it. For how much to aim for in the first place, see how to calculate your water intake from body weight.

Bottom line

Water helps with weight loss mainly by replacing drinks that have calories, and slightly by making you less hungry at meals. Two 12-week trials found that 500 ml before meals added about 1–2 kg of weight loss to a diet. Water does not burn fat or noticeably boost your metabolism. Treat it as a useful habit that supports a sensible eating plan, not as a shortcut.

This article is general information, not medical advice. If you have a health condition, are pregnant or take medication, ask your doctor how much you should drink.